join strings javascript a hands on guide to plus concat and join

Last updated: October 15, 2026

Most of us have been there — typing a greeting into a script, watching it come out as "helloundefined", and wondering what went wrong. Joining text in JavaScript looks simple until edge cases, performance quirks, and mixed data types get involved. We walked through the methods, weighed the trade-offs, and pulled together what actually matters for everyday coding.

According to shahzadmemon.com, JavaScript gives you several ways to glue strings together, and the differences between them are easy to miss. Picking the right one can save you debugging time, keep your code readable, and in some cases make your code noticeably faster.

Quick take

• Strings in JavaScript are immutable, so every "change" actually returns a new string.
• The + operator and += shorthand are the most intuitive way to combine strings.
• The concat() method is built specifically for joining two strings.
• The join() method collapses arrays of strings into a single string with a separator.
• Benchmark results vary wildly by machine, browser, and engine optimizations — measure your own code.

The four ways to join strings in JavaScript

The most familiar tool is the + operator. It works between literals, between variables, and even between strings and other data types when implicit conversion kicks in. According to the reference walkthrough, integers and booleans get coerced to their string form before being appended — so "hello" + 21 becomes "hello21", and "hello" + true becomes "hellotrue". The shorthand += is just a tidier version of the same idea, useful when you want to grow a string in place.

When you want something purpose-built, there is concat(). You call it on a string and pass in what you want appended. The reference also points out a less-known feature: you can pass a separator as the first argument, so "Java".concat(", ", "Script") prints "Java, Script". That comes in handy when you are stitching together labelled values for a log line or a UI string.

If your data is already sitting in an array, reach for join(). By default it inserts commas between elements, but you can pass any string — including an empty one — to control the output. The example in the source joins ["JavaScript", "HTML", "CSS"] with "" to get "JavaScriptHTMLCSS". This is the cleanest option whenever you are reading a file line-by-line or building a CSV-style blob from an array you already have.

Which method should you actually pick?

This is where the reference gets interesting. Shahzadmemon.com notes that some guides recommend concat() for performance, but the picture is messier than that. A simple benchmark looped many string operations two ways. On one machine, the + operator ran roughly twice as long as concat(). On another machine, the two finished almost identically. Running the same code on the first machine again flipped the result — the + operator won by a wide margin.

The reference also cites the MDN guidance, which actually recommends the assignment operators (+, +=) over concat(). The likely reason is that engines like V8 optimize repeated use of the same string literal in ways that micro-benchmarks do not capture. Comparing code like this, the source argues, is not as easy as just running it and watching the numbers — browser version, optimization passes, and even the specific strings you use all change the picture.

Our analysis: what it means for you

Here is the honest read. For the vast majority of code, the choice will not matter — readability should win. Use + or += when you are building a sentence, concat() when the intent really is "append this to that", and join() whenever you have an array in hand. The performance argument only matters inside tight loops processing very large numbers of strings, and even then, the right move is to measure on your own hardware rather than trust a generic benchmark.

Say you are building a status line in a small Node script that reads log lines from a file. You split each line into an array of fields with split(" "), you want to flip the order, and you need them back as a single string with dashes between each field. The cleanest path is arr.reverse().join("-"). That one line replaces a manual loop, sidesteps any coercion surprises, and reads exactly like what it does. If you had reached for + inside a loop instead, you would still get the right answer — just with more typing and a faint worry about whether it will hold up at scale.

Bottom line

Pick the method that matches the shape of your data. + and += for quick combinations, concat() for explicit appends, join() for arrays. Trust the MDN recommendation as a default, and only chase performance gains after you have measured real bottlenecks on the machines your code actually runs on.

Key takeaways

• Use + and += for simple, readable string building.
• Use concat() when you want an explicit append with an optional separator.
• Use join() whenever your data already lives in an array.
• Do not chase micro-benchmarks — measure on your own hardware.
• Follow MDN's default advice unless profiling proves otherwise.